This invention is generally concerned with methods of making anionic clays. Such clays are characterized by crystalline structures that consist of positively charged layers that are separated by interstitial anions and/or water molecules. The positively charged layers are often comprised of metal hydroxides of divalent metal cations (e.g., Mg2+, Ca2+, Zn2+, Mn2+, Co2+, Ni2+, Sr2+, Ba2+ and Cu2+) and trivalent metal cations (e.g., Al3+, Mn3+, Fe3+, Co3+, Ni3+, Cr3+, Ga3+, B3+, La3+ and Gl3+). The interstitial anions are usually NO3—, OH—, Cl—, Cr—, I—, CO32−, SO42−, SIO32−, HPO32−, MnO4—, HGaO32−, HVO42−, ClO4—, BO32−, monocarboxylates (e.g., acetate) and dicarboxylates (e.g., oxalate), alkyl sulphonates (e.g., lauryl sulphonate) and various combinations thereof.
Therefore, anionic clays are further subdivided according to the identity of the atoms that make up their crystalline structures. For example, anionic clays in the pyroaurite-sjogrenite-hydrotalcite group are based upon brucite-like layers (wherein magnesium cations are octahedrally surrounded by hydroxyl groups) which alternate with interstitial layers of water molecules and/or various anions (e.g., carbonate ions). When some of the magnesium in a brucite-like layer is isomorphously replaced by a higher charged cation, e.g., Al3+, then the resulting Mg2+—Al3+—OH layer gains in positive charge. Hence, an appropriate number of interstitial anions, such as those noted above, are needed to render the overall compound electrically neutral.
The literature also teaches that as the concentration of Al3+ increases in a Brucite-type lattice, a reduction of the lattice parameter known as “a”, takes place. The lattice parameter known as “c” also is reduced. The reduction in lattice parameter, a, is due to the smaller, plus three charged, Al3+ ions substituting for the larger, plus two charged Mg2+ ions. This higher charge causes increased coulombic forces of attraction between the positive charged Brucite-type layer and the negative interlayer ions—thus giving rise to a decrease in the size of the interlayer itself.
Natural minerals that exhibit such crystalline structures include, but by no means are limited to, pyroaurite, sjogrenite, hydrotalcite, stichtite, reevesite, eardleyite, mannaseite, barbertonite and hydrocalumite. The chemical formulas for some of the more common synthetic forms of anionic clays would include: [Mg6Fe2(OH)16]CO3.4H2O, [Mg6Al2—(OH)16]CO3.4H2O, [Mg6Cr2(OH)16]CO3.4H2O, [Ni6—Fe2(OH)16]CO3.4H2O, [Ni6Al2(OH)16]CO3.4H2O, [Fe4Fe2(OH)12]CO3.#H2O, [Ca2Al(OH)6](OH)0 75—(CO3)0 125.2.5H2O6]OH.6H2O, [Ca2Al—(OH)6]OH.3H2O, [Ca2Al(OH)6]OH.2h2O, [Ca2Al—(OH)6]OH, [Ca2Al(OH)6]Cl.2H2O, [Ca2Al(OH)6]0.5CO3.2.5H2O, [Ca2Al(OH)6]0.5SO4.3H2O, [Ca2—Fe(OH)6]0.5SO4.3H2O, [(Ni, Zn)6Al2(OH)16]CO3.4H2O, [Mg6(Ni, Fe)2(OH)16](OH)2.2H2O, [Mg6Al2(OH)16](OH)2.4H2O, [(Mg3Zn3)al2(OH)16]CO3.4H2O, [Mg6Al2(OH)16]SO4.xH2O, [Mg6Al2(OH)16](NO3)2.x-H2O, [Zn6Al2(OH)16]CO3.xH2O, [Cu6Al2(OH)16-]CO3.xH2O, [Cu6Al2(OH)16]SO4.xH2O and [Mn6Al2-(OH)16]CO3.xH2O, wherein x has a value of from 1 to 6.
Those skilled in this art also will appreciate that anionic clays are often referred to as “mixed metal hydroxides.” This expression derives from the fact that, as noted above, positively charged metal hydroxide sheets of anionic clays may contain two metal cations in different oxidation states (e.g., Mg2+ and Al3+). Moreover, because the XRD patterns for so many anionic clays are similar to that of the mineral known as Hydrotalcite, Mg6Al2(OH)16(CO3).4H2O, anionic clays also are very commonly referred to as “hydrotalcite-like compounds.” This term has been widely used throughout the literature for many years (see for example: Pausch, “Synthesis of Disordered and Al-Rich Hydrotalcite-Like Compounds,” Clay and Clay Minerals, Vol. 14, No. 5, 507–510 (1986). Such compounds also are often referred to as “anionic clays.” Indeed, the expressions “anionic clay,” “mixed metal hydroxides” and “hydrotalcite-like compounds” are often found very closely linked together. For example, in: Reichle, “Synthesis of Anionic Clay Minerals (Mixed Metal Hydroxides, Hydrotalcite),” Solid State Ionics, 22, 135–141 (1986) (at Paragraph 1, page 135) the author states: “The anionic clays are also called mixed metal hydroxides since the positively charged metal hydroxide sheets must contain two metals in different oxidation states. Crystallographically they have diffraction patterns which are very similar or identical to that of hydrotalcite (Mg6Al2(OH)16(CO3).4H2O); hence they have also been referred to as hydrotalcites or hydrotalcite-like.” (emphasis added). U.S. Pat. No. 5,399,329 (see col. 1, lines 60–63) contains the statement: “The term ‘hydrotalcite-like’ is recognized in this art. It is defined and used in a manner consistent with usage herein in the comprehensive literature survey of the above-referenced Cavani et al. article.”Hence, for the purposes of the present patent disclosure, applicant will (unless otherwise stated) use the term “hydrotalcite-like” compound(s) with the understanding that this term should be taken to include anionic clays, hydrotalcite itself as well as any member of that class of materials generally known as “hydrotalcite-like compounds.” Moreover, because of its frequent use herein, applicant will often abbreviate the term “hydrotalcite-like” with “HTL.”
The methods by which HTL compounds have been made are found throughout the academic and the patent literature. For example, such methods have been reviewed by Reichle, “Synthesis of Anionic Clay Minerals (Mixed Metal Hydroxides, Hydrotalcite),” Solid States Ionics, 22 (1986), 135–141, and by Cavani et al., CATALYSIS TODAY, Vol. 11, No. 2, (1991). In the case of hydrotalcite-like compounds, the most commonly used production methods usually involve use of concentrated solutions of magnesium and aluminum which are often reacted with each other through use of strong reagents such as sodium hydroxide, and various acetates and carbonates. Such chemical reactions produce hydrotalcite or hydrotalcite-like compounds which are then filtered, washed, and dried. The resulting HTL compounds have been used in many ways—but their use as hydrocarbon cracking catalysts, sorbents, binder materials for catalysts and water softener agents is of particular relevance to this patent disclosure.
It also is well known that HTL compounds will decompose in a predictable manner upon heating and that, if the heating does not exceed certain hereinafter more fully discussed temperatures, the resulting decomposed materials can be rehydrated (and, optionally, resupplied with various anions, e.g., CO3=, that were driven off by the heating process) and thereby reproduce the original, or a very similar, HTL compound. The decomposition products of such heating are often referred to as “collapsed,” or “metastable,” hydrotalcite-like compounds. If, however, these collapsed or metastable materials are heated beyond certain temperatures (e.g., 900° C.), then the resulting decomposition products of such hydrotalcite-like compounds can no longer be rehydrated and, hence, are no longer capable of forming the original hydrotalcite-like compound.
Such thermal decomposition of hydrotalcite-like compounds has been carefully studied and fully described in the academic and patent literature. For example, Miyata, “Physico-Chemical Properties of Synthetic Hydrotalcites in Relation to Composition,” Clays and Clay Minerals, Vol. 28, No. 1, 50–56 (1980), describes the temperature relationships and chemical identity of the thermal decomposition products of hydrotalcite in the face of a rising temperature regime in the following terms:                “It is of interest to know the form in which the Al occurs after thermal decomposition of the hydrotalcite structure. A sample with x=0.287, hydrothermally treated at 200° C. for 24 hr, was calcined at 300°–1000° C. in air for 2 hr. After calcination at 300° C., both hydrotalcite and MgO were detected by X-ray diffraction, but after calcination at 400°–800° C. only MgO could be detected. At 900° C. MgO, MgAl2O4, and a trace of γ-Al2O3 were detected.” (emphasis added, for reasons to be explained in the ensuing portions of this patent disclosure)Miyata then goes on to note that:        “The crystallite size was smaller than 50 Å when the sample was calcined below 800° C. This value was much smaller than that for MgO obtained from pure Mg(OH)2. On calcination above 800° C., the crystallite size rapidly increased. The changes of the crystallite size and lattice parameter a have the same tendency. Consequently, Al substituting in MgO acts to inhibit crystal growth. If Al-containing MgO is reacted with water, it should first form hydrotalcite. Hydrotalcite calcined at 400–800° C. with x=0.287 was hydrated at 80° C. for 24 hr, and the products were examined by X-ray powder diffraction. According to Table 7, hydrotalcite was the only hydrated product detected in samples calcined at 400–700° C. The lattice parameter a is the same as that of the original sample. The samples calcined at 800° C. also formed only hydrotalcite but their lattice parameters are larger than that of the original sample. According to FIG. 1, the molar ratio of this product is x=0.235. On the other hand, Al2O3 does not react with water under the above-mentioned conditions. Therefore, the results suggest that Al enters product MgO when hydrotalcite is calcined between 400 and 700° C.” (emphasis added)        
U.S. Pat. No. 5,459,118 (“the '118 patent”) describes the character of the materials that result from progressively heating hydrotalcite-like compositions (HTlc's) in a passage running from col. 4, line 67 to col. 5, line 14. It reads as follows:                “The natural products of calcination or activation in inert gas of a HTlc is believed to be a spinel. In the range between the temperature at which HTlc decomposition commences (between 572° and 752° F.) (i.e., between 300° C. and 400° C.) and that of spinel formation (1652° F.) (i.e., at 900° C.), a series of metastable phases form, both crystalline and amorphous. Therefore, the surface area, pore volume, and structure depend on the temperature of calcination. Upon calcination, the crystal structure of DHT-4A is decomposed at about 660° F. (i.e., 349° C.) when water and carbon dioxide evolved from the structure, and a MgO—Al2O3 solid solution of formula 4.5 MgO.Al2O3 is formed. This solid solution is stable up to 1472° F. (i.e., 800° C.) MgO and MgAl2O4 are formed at about 652° F. (i.e., 900° C.). On the other hand, the solid solution calcined at less than 1472° F. (i.e., 800° C.) can be restored in the original structure by hydration.” (The underlined portions of this passage have been added to convert ° F. to ° C. in order to more directly compare the teachings of this reference with other relevant references wherein temperatures are expressed in ° C., again such comparisons will be made in the next few paragraphs of this patent disclosure)        
It might also be noted here that this quotation from the '118 patent is a precise statement of the temperatures at which certain hydrotalcite decomposition products are described (e.g., spinel, MgAl2O4, formation taking place at 900° C. when hydrotalcite is thermally decomposed). This more exact knowledge of the temperatures at which certain aspects of the decomposition of hydrotalcite take place, clarifies many other, more general, statements found in the literature concerning the temperatures at which certain decomposition products are formed (e.g., statements concerned with the temperature at which spinel, MgAl2O4, is formed from a hydrotalcite starting material). That is to say that many, more general, statements concerning the temperatures at which various hydrotalcite thermal decomposition products (e.g., spinel, MgAl2O4) are formed must be carefully interpreted. For example, in U.S. Pat. No. 4,889,615 (“the '615 patent”) at col. 6, lines 36–43, we find the statement:                “Calcining the Mg/Al hydrotalcites at temperatures greater than 500° C. gives a mixture of MgO and MgAl2O4, a magnesium aluminate spinel, a material which has been reported to reduce FCC regenerator SOx emissions (see U.S. Pat. Nos. 4,469,589 (Yoo) and 4,472,267 (Yoo)). The activity of the dehydrated hydrotalcite is, however, significantly different than that observed for the spinel, MgO, or mixtures of both. No evidence of MgAl3812O4 (sic) is observed in the regenerated hydrotalcite indicating that the spinel is not the active component.” (emphasis added)        
Thus, in view of the previous, more precise, descriptions of the temperature of spinel formation (i.e., 900° C.) in the '118 patent, it seems that the more general expression “temperatures greater than 500° C.” used in the '615 patent should not be taken to mean something like 501° C., but rather should be taken to mean 900° C., a temperature which is indeed “greater than 500° C.” It also should be noted that the above-quoted passage recognizes that “spinel is not the active component” of the materials described in the '615. We note this point here because it is consistent with applicant's hereinafter described goal of not making spinel—so that applicant's heat treated, intermediate products can in fact be hydrated (or rehydrated) to form hydrotalcite-like compositions.
A similar general statement concerning spinel formation from a hydrotalcite precursor appears in U.S. Pat. No. 4,458,026. There (at col. 3, lines 54–56) we find the statement:                “Above 600° C. the resulting metal oxide mixture begins to sinter and lose surface area, pore volume, as well as form a catalytically inactive phase (spinel-MgAl2O4).” (emphasis added)        
Here again, applicant is of the opinion that the general expression “Above 600° C.” should not be taken to mean something like 601° C., but rather should be taken to mean far enough above 600° C. to form spinel—MgAl2O4 that is to say 900° C., the temperature at which spinel formation from a hydrotalcite-like compound has been more precisely determined. This quotation also notes that spinel is “catalytically inactive”.
Indeed, one can even find generalized statements about the temperature of spinel formation that are better interpreted to mean lower temperature levels. For example, in U.S. Pat. No. 5,114,898 (at col. 4, lines 43–51) we find the statement:                “Reichle in J. Catal. 101, 352 to 359 (1986) has shown that this heating of hydrotalcite was accompanied by an increase in the surface area from about 120 to about 230 m2/g (N2/BET) and a doubling of pore volume (0.6 to 1.0 cm3/g, Hg intrusion). Further heating to higher temperatures causes lowering of surface area as well as reactivity. At 1000° C., the formation of MgO and the spinel phase, MgAl2O4 has been observed.” (emphasis added)        
In this case, applicant thinks that the statement “At 1000° C. the formation of MgO and spinel phase has been observed”, is better taken to mean: spinel is observed at 1000° C. because spinel (MgAl2O4) forms at 900° C.—rather than taken to mean: 1000° C. is the temperature of formation of spinel. Indeed, applicant has by his own experimental work confirmed that spinel begins to from in HTL compounds at 900° C.
The prior art also has noted that when various anionic clay-forming ingredients such as hydrotalcite-forming ingredients (e.g., magnesium-containing compositions and aluminum-containing compositions) are mixed under certain prescribed conditions (e.g., certain aging times, pH conditions, temperatures, etc.), the resulting slurry or precipitate materials (e.g., hydrotalcite-like materials) will exhibit distinct catalytic properties. Hence, many such production processes are based upon fine tuning of such time, temperature, pH, etc. conditions in order to obtain maximum amounts of a given kind of hydrotalcite-like precipitate product.
The slurry and/or precipitate products of such initial chemical reactions also have been heat treated to obtain various “collapsed” or “metastable” hydrotalcite materials that have specific catalytic properties. Such collapsed materials have, for example, been used as sorbents (and especially SOx sorbents for fluid catalytic and fixed hydrocarbon cracking processes), hydrocarbon cracking catalysts, catalyst binders, anion exchangers, acid residue scavengers and stabilizers for polymers, and even as antacids intended for use in the context of human medicine.
The prior art also has long recognized that other ingredients such as compounds containing Ce, V, Fe and Pt can be added to the original hydrotalcite-forming reaction mixtures so they will appear as a distinct phase of various solid products created by such reactions. Dried forms of such anionic clays (e.g., microspheroidal particles of such hydrotalcite-like compounds used as SOx sorbents in fluid catalytic conversion (FCC) processes) also have been impregnated with solutions of such metals. Moreover, such metals have even been made a integral part of the crystalline structure of hydrotalcite-like materials (see, for example, U.S. Pat. No. 5,114,691 and U.S. Pat. No. 5,114,898 which teach use of sulfur oxidizing catalysts made of layered double hydroxide (LDH) sorbents, e.g., hydrotalcite-like materials that contain metal ions (e.g., those of vanadium) that replace some or all of the divalent metals (Mg2+) or trivalent metals (Al3+) that form the layers of the LDH).
Hydrotalcite-like compounds that are used as catalysts also have been both heat treated and associated with various catalyst binder or matrix materials. For example, U.S. Pat. No. 4,866,019 (the '019 patent) discloses that hydrotalcite can be heat treated and used in association with various binder materials. U.S. Pat. No. 5,153,156 teaches a method for making magnesium/aluminum synthetic anionic clay catalysts by (1) spray drying a slurry of a magnesium aluminum synthetic clay, (2) making a plasticized mixture of the spray dried clay with diatomaceous earth and (3) forming, drying and calcining the resulting plasticized mixture.
The prior art also has long recognized that anionic clay materials can be used to catalyze certain specific chemical reactions. For example, U.S. Pat. No. 4,458,026 teaches use of certain heat treated anionic clay materials as catalysts for converting acetone to mesityl oxide and isophorone. The anionic clays are given this catalytic activity by heating them to temperatures ranging from about 300 to 600° C.
U.S. Pat. No. 4,952,382 teaches a hydrocarbon conversion process that employs a catalyst composition containing an anionic clay wherein the anionic clay serves as a sulfur oxides binding material.
U.S. Pat. No. 4,970,191 teaches use of polymorphic-magnesium-aluminum oxide compositions as catalysts in various base catalyzed reactions such as alcohol condensation, isomerization of olefins, etc.
U.S. Pat. No. 4,889,615 discloses a vanadium trap catalyst additive comprising a dehydrated magnesium-aluminum hydrotalcite.
U.S. Pat. No. 5,358,701 teaches the use of layered double hydroxide (LDH) sorbents such as hydrotalcite-like materials as SO2 sorption agents. This reference postulates that the sulfur-containing gas absorbs into the hydrotalcite structure as SO32− anions by replacing the gallery CO32− anions. The absorbed sulfur is thereafter driven off by calcination at elevated temperatures (500° C.). The LDH sorbents are regenerated by hydrolyzing the calcined product, particularly in the presence of CO2 or CO32−.
U.S. Pat. No. 5,114,691 teaches removing sulfur oxide from gas streams using heated layered double hydroxide (LDH) sorbents having metal-containing: oxoanions incorporated into the galleries of the LDH structures.
U.S. Pat. No. 4,465,779 teaches catalytic cracking composition comprising a solid, cracking catalyst and a diluent containing a magnesium compound in combination with a heat-stable metal compound.
U.S. Pat. No. 5,426,083 teaches catalytic use of a collapsed composition of microcrystallites comprised of divalent metal ions, trivalent ions, vanadium, tungsten or molybdenum.
U.S. Pat. No. 5,399,329 teaches making hydrotalcite-like materials by preparing a mixture of magnesium (divalent cation) to aluminum (trivalent cation) in a molar ratio between 1:1 and 10:1, and in a mono carboxylic anion to aluminum (trivalent cation) molar ratio between 0.1:1 to 1.2:1. The process involves reacting a mixture comprising magnesium and aluminum cations and mono carboxylic anions in an aqueous slurry having a temperature of at least 40° C. and a pH of at least 7. Generally speaking, a given synthesis of a HTL compound by any of the methods taught in these patents was considered a success when the product of its chemical synthesis reaction (slurries typically were heated and/or pressured to form a final dry product or precipitate) produces a given HTL compound having an x-ray diffraction pattern which reasonably resembles that of a given card in the files of the International Center for Diffraction Data (“ICDD”)
In summarizing the prior art, it might be said that most methods that have been employed to produce anionic clay compounds, and especially hydrotalcite-like, anionic clay compounds, usually involve precipitation or slurry drying of a hydrotalcite-like product, washing and, optionally, heat treatment of the resulting dried slurry, or precipitated, composition. Once made, these HTL compounds, or their thermal decomposition products, have been employed as catalysts (e.g., as vanadium passivators, SOx additives, aldol condensation catalysts, water softening agents, and even medicines).